The HIV-retrovirus is the etiologic agent for AIDS. The high level of viral expression is attributed, at least in part, to the virus- encoded transactivator protein TAT, which has the ability to autoregulate the expression of HIV by promoting the transcriptional activity of the HIV LTR promoter. The molecular cloning of HIV has provided the isolation of TAT coding DNA and LTR promoter sequence from other HIV sequences and has facilitated studies of the TAT protein on the LTR promoter independent of other retroviral proteins. In the past year, we used an in vitro cell-free transcriptional system to study the transacting activity of the TAT protein on the HIV LTR promoter and other virus and cellular gene promoter. The effect of the TAT protein on the expression of genes directed by different promoter were compared by using nuclear extract of HeLa/t2 cell, which constitutively expresses TAT protein. We found that the HeLa/t2 nuclear extract markedly enhanced the transcription from HIV LTR promoter, but did not show any enhancement of transcription from beta-globin, epsilon-globin and adenovirus MLP promoters. Furthermore, when various amounts of HeLa/t2 nuclear extract were added to the transcriptional reaction mixture, the amount of the specific RNA product from HIV LTR was increased proportionally. The results, therefore, suggest that the TAT protein may be a specific trans-activating factor for the HIV LTR promoter. Further studies with purified TAT protein from prokaryotic cell, using cloned TAT gene in expression vectors, is being done to be able to provide directly evidence for the transacting activity of TAT protein and to develop assays for the study of potential inhibitors of the protein. Large scale production, isolation and purification of the tat protein from prokaryotic cells using cloned tat DNA has yielded a small amount of tat-protein (a few micrograms). However, the method of production has resulted in extensive degradation of the protein product. This work could lead to a new approach to the prevention or treatment of AIDS.